Project Summary/Abstract Despite advances in treatment and screening, colorectal cancer (CRC) remains the third most common cancer and fourth most common cause of cancer-related death worldwide. Although the role of selenium (Se) in cancer remains controversial, several epidemiological studies have revealed inverse correlations between CRC risk and Se nutritional levels. Additional molecular studies have demonstrated that Se deficiency worsens inflammatory- driven colitis-associated cancer (CAC) and stimulates the Wnt signaling pathway, which is inappropriately activated in the vast majority of CRCs. Se is an essential trace element incorporated into selenoproteins (SEPs) as selenocysteine (SEC). As both the most highly expressed SEP and the only SEP with multiple SECs, SEPP1 is hypothesized to supply Se to various tissues for production of the other 24 SEPs in the genome. In addition to its Se transport capability, SEPP1 possesses antioxidant activity via its redox domain. Collectively, such functions purport a cancer-preventive role for SEPP1. In fact, SEPP1 mRNA expression is downregulated in CRCs. Moreover, global SEPP1 reduction promoted tumorigenesis and activated Wnt signaling in a CAC animal model. SEPP1 is primarily synthesized in the liver and delivered to distant tissues through the plasma. Surprisingly, we observed that liver-specific Sepp1 deletion did not impact colitis severity nor subsequent tumorigenesis in a murine CAC model. These results suggest a protective role for locally-produced SEPP1. Interestingly, Sepp1 deletion specifically from the intestinal epithelium phenocopied global SEPP1 reduction and decreased tumorigenesis in the same CAC model. Moreover, dietary Se supplementation attenuated tumorigenesis and promoted survival in Sepp1-deficient mice. While these results imply that Se and SEPP1 modify inflammatory-driven CAC, little is known about the role of epithelial SEPP1 in non-inflammatory-driven (and much more prevalent) sporadic CRC and, importantly, if dietary Se can compensate for genetic SEP deficiencies to protect against tumorigenesis. To determine how SEPP1 modulates non-inflammatory-driven sporadic CRC, I will investigate the impact of SEPP1 on initial tumor formation as well as on established colorectal tumor growth. Specifically, I will analyze the effects of intestinal epithelial SEPP1 loss on tumorigenesis and Wnt signaling under deficient, sufficient, and supranutritional Se levels. Additionally, I will delineate the contributions of tumor- and host-derived SEPP1 to established colorectal tumor growth. Lastly, I will examine the consequences of SEPP1 expression on apoptosis, DNA damage, in vivo growth, oxidative stress, proliferation, and Wnt activation using ex vivo 3D-organoid cultures of human primary CRC. Together, these experiments will help elucidate the roles of Se and SEPP1 in sporadic colorectal carcinogenesis.